Anahim Hotspot - Petrology and Geochemistry

Petrology and Geochemistry

Analysis of the chemical composition of the lavas gives important clues about the source and dynamics of the hotspot plume. Where hotspots occur under thick continental crust, basaltic magma is trapped in the less dense continental crust, which is heated and melts to form rhyolites. These rhyolites can be quite hot and form violent eruptions, despite their low water content. Such rhyolitic magma can be found at the western part of the Anahim Volcanic Belt, which contrasts the more basaltic material in the Nazko Cone area. This suggests that as the North American Plate moves westwards, the Anahim hotspot underlies thinner continental crust. This hypothesis has been verified by observation of the crustal regime in other compression margins – the thickest granitic structures are found near the margin itself, with the North American Plate becoming less compressed in regions away from the margin. In this case, the thinner crust would represent briefer travel time, thus reducing the time available for magma differentiation, whose end products are rhyolites. As most magma is basaltic in origin, the eruption would therefore contain more basaltic materials. A few igneous rock types with composition unlike basalt, such as nephelinite, do occur at the small basaltic cinder cones and flows but are extremely rare.

Basaltic lava flows have a high ferromagnesian (iron and magnesium) content and erupt at temperatures between 1000°C and 1200°C; these values are higher than those of other common igneous rocks. Due to the high temperature of this lava flow, the lava would be extremely fluid (it has low viscosity), allowing the lava to travel long distances from the magma source (the volcano or vent). These extremely fluid lavas have flow speeds that depend heavily on underlying terrain, with a maximum of almost 60 kilometres per hour (37 mph) in underground lava channels. Flow independent of such channels and tubes moves quite a bit slower, averaging speeds of 1.6 km (1 mi) per hour. However, this flow speed changes considerably within the flow, with speeds depending heavily on depth and degree of cooling that the flow has experienced (essentially distance from the vent). Although the Nazko area flows would not be highly basaltic like those found in Iceland or Hawaii, lower depths are unlikely as viscosity is normally higher.

A few volcanic centers in the vicinity of the Bella Bella and Gale Passage dike swarms, such as Helmet Peak and Kitasu Hill, which are members of the Milbanke Sound Group, may represent the westernmost of the Anahim volcanoes, but their ages are significantly different, provoking on-going questions about their origin and connection to other regional volcanic activity. However, many volcanoes in that center are believed to be monogenetic, suggesting a monogenetic volcanic field may be responsible.

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